In the drilling and production of oil and gas wells, a well bore is drilled by means of a plurality of drill pipes provided in sufficient numbers to assemble a rotatable drill string sufficient to drill the required depth. The rotatable drill string is terminated by a drill bit and typically provided with stand-off stabiliser parts periodically throughout the length of the drill string. The drill string is rotated to remove formation ahead of the drill bit, to drill out and thus form a wellbore, and to increase the depth of the well. Drilling mud or other fluid is circulated through the drill string to cool, lubricate and clear the drill bit of cuttings, and to displace the resulting drill cuttings from the bottom of the well to the surface, via an annulus formed between the drill string and the wall of the wellbore.
Periodically, the drill bit is removed from the wellbore and a casing string comprising lengths of tubular casing sections coupled together end-to-end is run into the drilled wellbore and cemented in place. A smaller dimension drill bit is then inserted through the cased wellbore, to drill through the formation below the cased portion, to thereby extend the depth of the well. A smaller diameter casing is then installed in the extended portion of the wellbore and also cemented in place. If required, a liner comprising similar tubular sections coupled together end-to-end may be installed in the well, coupled to and extending from the final casing section. Once the desired full depth has been achieved, the drill string is removed from the well and then a workstring is run-in to clean the well. Once the well has been cleaned out, the walls of the tubular members forming the casing/liner are free of debris so that when screens, packers, gravel pack assemblies, liner hangers or other completion equipment is inserted into the well, an efficient seal can be achieved between these devices and the casing/liner wall.
The step of cleaning the wellbore is usually achieved by inserting a workstring containing dedicated well clean-up or cleaning tools. Typical well cleaning tools known for use in this environment include scrapers, wipers and/or brushes which are held against the internal wall of the casing/liner, to clean away debris as the tool is run-in and then pulled out of the wellbore.
One such type of well cleaning apparatus would be a casing scraper, i.e. a tool whose working surface elements incorporate steel casing scraper blades that scrape the inside of the casing or tubing in the well. The steel blades provided with casing scrapers usually are designed to clean the casing interior of relatively large particles or debris, such as lumps of cement, rocks or caked mud.
A second type of well cleaning apparatus known in the art may be more accurately likened to a brush and the working surface elements thereof incorporate cleaning pads with protruding bristles. Brushing tools are generally used to clean well casings, tubing and the like of smaller debris and or particles than that of scraper tools. Brushing tools may be used to remove oxidation lumps, scale, paraffin and metal burrs for example.
It is also possible to augment the cleaning action of such tools by including means within the tool or string for re-directing fluid flow from a circulation path through a workstring to impinge upon a particular area or zone within the wellbore. The re-directed flow of circulation fluid may be routed via ducts and outlet ports to flush around a workstring in proximity to a mechanically scraped or brushed surface to facilitate return of dislodged detritus in the circulation fluid to surface or otherwise for recovery in a trap within the string.
A workstring for use in cleaning a wellbore casing or liner may include any number of special purpose tools to allow several cleaning activities to be performed in a single trip, but traditional practice requires that a drill string is withdrawn and a workstring (i.e. a string lacking a BHA) is run in hole to perform the cleaning or remedial activity. Downhole tasks may be accomplished using a “workstring” made up of stands of drill pipe, tubing, coil tubing or wireline.